Failure of successful axon regeneration in the CNS is attributed not only to the intrinsic regenerative incompetence of mature neurons, but also to the environment encountered by injured axons. The inhibitory activity is principally associated with components of CNS myelin and chondroitin sulfate proteoglycans (CSPGs) in the glial scar at the lesion site (1-4). Recent studies suggested that three myelin proteins, myelin-associated glycoprotein (MAG), Nogo-A and oligodendrocyte myelin glycoprotein (OMgp), collectively account for the majority of the inhibitory activity in CNS myelin (4-6). The inhibitory activity of MAG, OMgp and the extracellular domain of Nogo-A may be mediated by a common receptor complex that consists of the ligand-binding Nogo-66 receptor (NgR) and its signaling co-receptors p75/TROY and Lingo-1 (7-13). However, little is known about how signaling events occurring at the axonal membrane are translated into specific cytoskeletal rearrangements underlying inhibition of axon regrowth. For instance, it is known that MAG and perhaps other myelin inhibitors are able to induce an elevation of intracellular Ca2+ levels (14-16). But it is unclear how intracellular Ca2+ signaling may be involved in the inhibition of axon regeneration.
The involvement of EGFR activation in development and differentiation of CNS neurons has been studied extensively. Goldshmit et al. (J Biol Chem (2004) 279:16349-16355) report that overexpression of SOCS2 in CNS neurons promotes neurite outgrowth, and that this outgrowth is blocked by addition of EGFR inhibitors PP3 and AG490. Wu et al. (Mol Biol Cell (2004) 15:2093-2104) report that the chondroitin sulfate proteoglycan versican V1 induces NGF-independent neuronal differentiation and promotes neurite outgrowth in cultured PC12 cells by enhancing EGFR and integrin activities, and that addition of the EGFR inhibitor AG1478 significantly blocks differentiation. Wildering et al. (J Neurosci (2001) 21:9345-9354) report that EGF promotes axonal regeneration of neurons of the crushed right internal parietal (RIP) nerve in the pond snail Lymnaea stagnalis and that inhibition of EGF action by the specific EGFR inhibitor PD153035 counteracts the effect of EGF on axonal regeneration. Li et al. (J Neurosci (2003) 23:6956-6964) report that PC12 cell lines with reduced EGFR signaling have reduced neurite outgrowth in response to NGF and that AG1478, a specific EGFR tyrosine kinase inhibitor, is cytotoxic to these cells.
In light of these reports our finding that suppressing EGFR function promotes significant regeneration of a lesioned adult CNS neuron in the presence of myelin inhibitory molecules was quite unexpected.